Catholics do not ask Mary to replace Christ. They ask her to pray for them because she belongs to Christ completely and leads souls to him. To ask for Mary's intercession is to ask the Mother of the Lord to join her prayers to ours.
Christ is the one mediator between God and man, and every grace comes from him. Catholic devotion to Mary does not compete with this truth. It depends on it. Mary's intercession has power only because of Christ, never apart from him. That is why Catholics do not approach Mary as an alternative to Jesus, but as the one who most perfectly says, "Do whatever he tells you."
Christians already ask one another for prayer. The Catholic understanding is that death does not sever the communion of saints. Those who are alive in Christ remain members of his Body. If we ask fellow Christians on earth to pray for us, it is fitting also to ask for the prayers of those who now live with the Lord in glory.
Mary's place is unique because she is the Mother of Jesus. She said yes to the Incarnation, pondered the mysteries of Christ in her heart, stood at the foot of the Cross, and was given to the beloved disciple by Jesus himself. Catholic tradition has therefore long seen in John 19:26-27 a sign of her maternal care for the disciples of Christ.
To ask Mary's intercession is not to stop at Mary. It is to let her accompany us to her Son. She forms souls in Christ by teaching them obedience, humility, recollection, and trust. Her maternal role is real, but always subordinate to his saving work.
The Rosary expresses this beautifully. The repeated Hail Marys do not trap the soul in Mary, but move the soul through Mary into deeper contemplation of Jesus Christ. In this way, true Marian devotion remains Christ-centered.